This invention concerns stable ohmic contacts for, and a process for making stable ohmic contacts to, thin films of p-type tellurium-containing II-VI semiconductors and electronic devices, particularly photovoltaic devices, incorporating the novel stable contacts.
Photovoltaic devices incorporating layers of tellurium-containing II-VI semiconductors offer substantial promise in terms of conversion efficiency, cost and ease of manufacture. As used here, the term tellurium-containing II-VI semiconductors refers to semiconductors containing tellurium and one or more elements selected from Group IIB of the Periodic Table of the Elements, as published by the American Chemical Society, including mercury cadmium telluride, cadmium telluride, cadmium zinc telluride, mercury zinc telluride, zinc telluride and mercury telluride. Of these semiconductors, cadmium telluride, mercury cadmium telluride, cadmium zinc telluride and zinc telluride offer particular promise. Of the II-VI tellurium-containing semiconductors, cadmium telluride and mercury cadmium telluride have probably been the subject of the most research. These materials can be prepared by a number of techniques. For example, monocrystalline cadmium telluride can be prepared by the Bridgeman technique. Thick films of cadmium telluride can be prepared by sintering a screen printed, dried paste of cadmium and tellurium particles. These thick films, e.g. 20 micrometers thick, are sintered at rather high temperatures (about 600.degree. to 700.degree. C.) to react the particles of cadmium and tellurium to form cadmium telluride. Thin films of cadmium telluride can be conveniently electrodeposited as disclosed in U.S. Pat. Nos. 4,400,244 and 4,425,194. Thin films of cadmium-rich mercury cadmium telluride can be electrodeposited following the disclosure of U.S. Pat. No. 4,548,681. The electrodeposited cadmium telluride-containing thin films generally exhibit n-type conductivity, but may be converted to p-type conductivity by heat treating in accordance with the disclosure of U.S. Pat. No. 4,388,483. An ohmic contact to electrodeposited thin films of cadmium telluride-containing semiconductors may be prepared according to the disclosure of U.S. Pat. No. 4,456,630. The disclosures of the cited patents are incorporated herein by reference.
While ohmic contacts to crystals and films of p-type tellurium-containing II-VI semiconductors have been successfully made by various processes, including by the method disclosed in U.S. Pat. No. 4,456,630, we have observed that some ohmic contacts to thin films of p-type tellurium-containing II-VI semiconductors do not remain stable over time. For example, gold contacts on electrodeposited cadmium telluride and mercury cadmium telluride that were prepared according to the '630 patent have been observed to react with or diffuse into the tellurium-containing semiconductor. As discussed below, previously known copper-nickel contacts have been observed to form flakes that detach from the semiconductor. In order to prepare photovoltaic devices containing thin films of p-type tellurium-containing II-VI semiconductors that are useful over reasonable lifetimes, ohmic contacts having long term stability must be easily and cheaply prepared.
Accordingly, it is a principal object of this invention to prepare ohmic contacts to thin films of p-type tellurium-containing II-VI semiconductors that are stable over a long period of time.
It is another object of this invention to make an ohmic contact to a thin film of p-type tellurium-containing II-VI semiconductors requiring only simple, low cost processing steps and materials.
It is a further object of this invention to achieve a photovoltaic device including a thin film of a p-type tellurium-containing II-VI semiconductor and incorporating a stable ohmic contact.